Decompression monitor



Aug. 26, 1969 GULINO ETAL DECOMPRE SSION MONITOR Filed Dec. 12, 1966 5Sheets-Sheet 2 INVEN'IORS ROSARIO GULINO 8 KENNETH H. HALL ,w 4%w44their ATTORNEYS Aug. 26, 1969 R. GULINO ETAL DECOMPRESSION MONITOR 5Sheets-Sheet 5 Filed Dec. 12. 1966 FIG. 6

INVIZN'IORS ROSARIO GULINO 8 KENNETH H. HALL 5M, By, dw [Ono-LL.

their ATTORNEYS United States Patent 3,463,015 DECOMPRESSION MONITORRosario Gulino and Kenneth H. Hall, Ledyard, Conn.,

assignors to General Dynamics Corporation, New York,

N.Y., a corporation of Delaware Filed Dec. 12, 1966, Ser. No. 601,083Int. Cl. G01n 33/00 [1.8. CI. 73-432 12 Claims ABSTRACT OF THEDISCLOSURE A monitor is provided for a diver indicating to him acontinuous rate of ascent after exposure to high hydrostatic pressures.Flow barriers within the monitor permit passage of fluid at a ratedependent upon the ambient pressure and the length of time the diver hasbeen down in the water, creating a mechanical analogy to the absorptionof inert gas in the divers bodily tissues. As the diver ascends, theindication of the comparative pressures permits the diver to ascend atthe fastest rate permissible to avoid decompression syndrome, or bends.

This invention relates to diving apparatus and, more particularly, toinstruments which automatically compute decompression schedules.

When a diver descends into the sea, the extra air pressure to which heis subjected is instantly transmitted to the inside of his body.Consequently, the diver breathes the breathing mixture at a pressurehigher than atmospheric and at each breath, a certain amount of nitrogenor some other inert gas is dissolved in the lungs. The body tissues, intheir turn fed by the blood, are charged with the inert gas in an amountdependent upon the duration of the dive and the hydrostatic pressure. Onreturning to the surface, the process is reversed. The excess gasdissolved in the different tissues is carried by the blood to the lungsand then eliminated in respiration. If the rise is too rapid, thedifierence between the pressure of the inert gas dissolved into thetissues and the hydrostatic pressure is such that bubbles form and it isthese bubbles which cause depression syndrome or bends. Moreparticularly, decompression syndrome results whenever the tissue ratioof any individual body tissue is exceeded, the tissue ratio beingexpressed as the ratio between the pressure of the gas dissolved in thetissues and the hydrostatic pressure to which the tissue is exposed.

One technique designed to calculate the safe rate of ascent by thedriver involves the use of tables and equations wherein the compressionand decompression history of representative body tissues are computedseparately and compared to the hydrostatic pressure. While reliable,manual calculations of the exact pressure-time ascent trajectory areextremely arduous and time-consuming. Moreover, once the calculationsare made, the rate of ascent is still restricted in that safety factorsare applied to the results of the calculations in order to avoid anypossibility of decompression syndrome.

Attempts have been made to construct decompression monitors whichautomatically compute the trajectory of safe ascent. One such deviceincludes a single time constant diffusion barrier which is employed toanalog all the tissues of the human body. It is apparent that thegrouping of all the body tissues into a representative diffusion barrieris wholly inadequate since decompression syndrome results whenever thetissue ratio of any body tissue is exceeded.

It is an object of the present invention, accordingly, to provide adecompression monitor which provides information for the establishmentand verification of the parameters used in decompression tables andequations.

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It is another object of the present invention to provide a decompressionmonitor which eliminates restricted ascent by indicating a minimal timeascent trajectory for preparameters used in decompression tables andequations.

It is a further object of the present invention to provide adecompression monitor which generates information for the evaluation ofthe pressure safety margin of a given decompression procedure.

These and other objects are accomplished by providing a decompressionmonitor which is responsive to several components, each of which isarranged to represent the recent pressure history of representative bodytissues, the various body tissues each having a maximum permissibleratio of the partial pressure of absorbed gas to hydrostatic pressure.In order to avoid decompression syndrome resulting when the ratio ofpartial pressure to hydrostatic pressure exceeds the above-defined ratiofor any tissue, the decompression monitor automatically computes theminimum permissible hydrostatic pressure for the most susceptible bodytissue and provides an indication of the maximum further decompressionor ascent permissible continuously in time.

Further objects and advantages of the present invention will be apparentto those skilled in the art from a reading of the following descriptionof the invention taken in conjunction with the following drawings, inwhich:

FIGURE 1 is a schematic block diagram illustrating the arrangement of atypical decompression monitor according to the invention;

FIGURE 2 is a sectional illustration of one of the body tissuesimulators coupled to the pressure multiplier of the decompressionmonitor shown in FIGURE 1;

FIGURE 3 is a sectional view showing one of the pressure measuring unitsof the monitor;

FIGURE 4 is a digrammatic representation of the electrical circuit usedin the permissible ascent indicator of the monitor;

FIGURE 5 illustrates one type temperature compensated flow barrier foruse in the representative body tissue simulator of FIGURE 2; and

FIGURE 6 illustrates another type flow barrier for use in therepresentative body tissue simulator of FIGURE 2.

As represented in the block diagram of FIGURE 1, a decompression monitoraccording to the present invention includes a pressure multiplier 4 forapplying a pressure corresponding to the product of the hydrostaticpressure and the percentage of inert gas in a breathing mixture througha pressure-transmitting conduit 5 to several body tissue simulators 6,6', 6 comprising pressure accumulators arranged in the manner describedhereinafter to simulate representative body tissues. Each tissuesimulator is connected by a pressure-transmitting conduit 7, 7, 7" to acorresponding pressure detector 8, 8, 8" which responds to pressuresapplied from the related tissue simulator to transmit correspondingsignals to a permissible ascent indicator 9. This indicator selects fromthe various signals applied to it, the one representing the lowestpermissible ascent, and provides a corresponding visual indication. Itwill be understood that the monitor illustrated schematically in FIG. 1is preferably embodied in a portable unit carried by a diver and mayinclude more or fewer tissue simulators and pressure detectors ifdesired. In this regard, since each body tissue is characterized by atime constant which is the rate of gas elimination or absorption plottedagainst time, the time constants may be represented by exponentialcurves, each curve exhibiting a different gradient. Each pressureaccumulator 6, 6', 6" corresponding to a particular body tissuetherefore is characterized by an exponential curve, and because of thevariety of tissues in the body having different characteristics, thepreferred embodiment of the invention shown in FIG. 1 includes threeaccumulators, the characterizing curves of each having differentgradients.

All of the tissue simulators have the same structure and, as illustratedin FIGURE 2, the simulator 6 comprises a pressure accumulator housing 10having a rigid outer wall 12 which flares out into a partial pressurechamber 14 and a flexible diaphragm 16 located within the chamber 14.This diaphragm is secured at its ends between the outer wall 12 and apair of sealing members 18 and 20 which prevent liquid leakage along theedges of a fluid flow barrier 22. The fluid flow barrier 22 comprisesany arrangement permitting fluid to pass in proportion to the pressuredifference across the barrier and is interposed between the diaphragm 16and an inner accumulator chamber 24. The accumulator chamber isconnected to the pressure detecting assembly of FIGURE 3 through apassageway 26 and the conduit 7 and has its volume defined by a springbiased piston 28 and the barrier 22.

A fluid, such as, for example, Dow Corning 200 Silicone fluid having aviscosity of 500 centistokes at C., fills the diaphragm 16 and passesthrough the barrier 22 at a rate which is proportional to the differencein pressure between the fluid in the diaphragm 16 and the fluid whichhas passed into the chamber 24. Moreover, the rate of liquid pressurebuildup in the chamber 24 is further dependent upon the time constant ofthe accumulator which is determined by the viscosity of the fluidcontained in the diaphragm 16, the porosity of the fluid flow barrier 22and the volume of the chamber 24. It is necessary that the time constantof the accumulator approximate as nearly as possible the time constantof the particular body tissue which the accumulator simulates. To thisend, the spring constant of the piston 28 is chosen such that the volumeof the chamber 24 increases slightly with increasing pressure and thefluid flow barrier 22 is designed such that the liquid flow across thebarrier remains constant over a wide temperature range. The fluid flowbarriers illustrated in FIGURES 5 and 6 are of such design and maintaina uniform flow rate, as will be described further below.

In a typical breathing mixture, the percentage of inert gas present mayvary between 5% and 95%. In order to take this percentage into account,the pressure chamber 14 is filled with a fluid, such as the Dow CorningSilicone mentioned above, and connected to the pressure multiplier 4which, as mentioned above, applies a pressure through the conduit 5 tothe fluid of chamber 14 corresponding to the product of the hydrostaticpressure and percentage of inert gas.

The multiplier comprises a housing 30 and a pair of levers 32 and 34pivotably mounted on opposite sides of the housing 30 and connected to apair of pistons 36 and 38, the piston 36 engaging liquid at thehydrostatic pressure and the piston 38 engaging the liquid in thepressure chamber 14. Further included within the multiplier 5 are afulcrum roller assembly 40 for slidably engaging levers 32 and 34 and acontrol knob 42 for positioning the assembly 40 and for impartingtranslational movement to the pivotably mounted arm 44 of a percentageinert gas indicator 46 which includes markings from 0 to 100.

In order to pressurize the fluid of chamber 14, as well as the partialpressure chambers of the tissue simulators 6' and 6", to a value whichcorresponds to the percentage of inert gas in the breathing mixture andthe hydrostatic pressure, the knob 42 is adjusted until the indicator 46affords a percentage inert gas reading which coincides with the actualpercentage inert gas. For greater percentages of inert gas, as indicatedby the meter 46, greater percentages of the hydrostatic pressure exertedagainst the piston 36 are correspondingly translated into pressureforces exerted on the liquid in the chamber 14 by the piston 38. If, forexample, the diver descended to a depth of 198 feet, he would besubjected to a hydrostatic pressure of 88.1 lbs. per square inch(p.s.i.). If his breathing mixture contained 50% nitrogen, the indicator46 would be set to read 50%, which setting would place the fulcrumroller assembly midway between the center points of the pistons 36 and38. The force exerted against the lever 32 by the piston 36 would betranslated to the piston 38 by the lever 34 in a ratio of two to onesuch that the pressure of the liquid in the chamber 14 would amount to44.05 p.s.i. If the indicator 46 were set to read the pressure of theliquid in the chamber 14 would correspondingly increase to a value of72.5 p.s.i. for the same value of hydrostatic pressure.

As indicated in FIGURE 1, the decompression monitor further includesthree pressure detectors 8, 8' and 8", each being associated with acorresponding tissue simulator. FIGURE 3 illustrates in detail thestructure of the pressure detector 8, the others being identical inarrangement. As shown in FIGURE 3, the pressure detector 8 comprises abalancing mechanism arranged to provide an output signal indicative ofthe diflerence between the hydrostatic pressure of the water at thedepth to which the diver has descended and the critical hydrostaticpressure as determined by the tissue ratio of the simulated body tissue.The balancing mechanism includes a housing 50 having located therein anaccumulator pressure chamber 52 connected through the conduit 7 to thechamber 24 of the accumulator and a hydrostatic pressure chamber 54exposed to the ambient water or to a liquid at the hydrostatic pressure.

The pressure of the liquid in the accumulator chamber 52 is convertedinto a proportional static force by acting on a diaphragm 56 which isconnected to the chamber 52 via a bellows 58. Similarly, the hydrostaticpressure is converted into a proportional static force by acting on adiaphragm 60 which is connected to the chamber 54 through a bellow 62.It is noteworthy that the opposite sides of the diaphrgam 56 and 60 areexposed to a vacuum, thus providing forces proportional to absolutepressure. Each static force is transmitted to opposite ends of a balancebeam 62 by a pair of push rods 64 and 66 whose lengths are adjusted by apair of sleeves 68 and 70, respectively, such that minimal contact ismade between the push rods and the diaphragms 56 and 60 when the beam 62is in the balanced condition and the bellows 58 and 60 are in therelaxed position. Moreover, adjustable mechanical stops 72 are providedand place along the rod 64 for limiting the travel of the rod for anunbalanced condition exceeding the range of interest.

The balance beam 62 engages a fulcrum 74 which is positioned along thebeam 62 by means of a pair of adjustable screws 76 and 78 in accordancewith the tissue ratio of the simulated body tissue. While the areas ofthe diaphragms 56 and 60 may be varied in order to change the tissueratio, in the preferred embodiment of the invention, the areas of thediaphragms are maintained equal and only the fulcrum 74 is varied toachieve the tissue ratio of the simulated body tissue. For example, atypical body tissue may have a maximum permissible ratio of the partialpressure of absorbed gas to hydrostatic pressure of 1.80. The fulcrum 74would be positioned along the beam 62 such that a balance conditionwould exist whenever the force exerted by the pressure of liquid in theaccumulator chamber 52 against the diaphragm 56 is 1.8 times the forceexerted by the pressurized liquid in the chamber 54 against thediaphragm 60.

As mentioned above, the tissue ratio for any tissue varies as a functionof the hydrostatic pressure, decreasing slightly at great depths,increasing slightly at small depths. In order to compensate for thisvariance, the balancing mechanism further includes a spring-biasedpressure responsive bellows 80 exposed to liquid at the hydrostaticpressure and connected to the fulcrum adjust screw 78 through adiaphragm 82. This diaphragm imparts a force against an opposingadjustable spring bias mechanism 84 so as to move the fulcrum 74 inpropor tion to the hydrostatic pressure and the setting of the bias.Thus, the tissue ratio as determined by the positioning of the fulcrum'74 decreases with increasing hydrostatic pressure and increases withdecreasing hydrostatic pressure.

The relative motion of the push rods 64 and 66 may therefore beexpressed in terms of the difference between the ambient hydrostaticpressure and the hydrostatic pressure required for a balanced condition.Coupled between the push rods 64 and 66 is a rotary balance beampotentiometer 86, having a movable contact 88 which provides anelectrical signal indicative of the difference between the hydrostaticpressure of chamber 54 and the pressure of the liquid in the chamber 52.When the pressure of the liquid in the chamber 54 is greater than thepressure of the liquid in the chamber 52 divided by the tissue ratio,the push rod 64 exerts a translational movement against thepotentiometer 86 such that the effective resistance of the potentiometerincreases. When the pressure of the liquid in the chamber 54 is lessthan the pressure of the liquid in the chamber 52 divided by the tissueratio, the push rod 66 exerts a translational movement against thepotentiometer 86 such that the effective resistance of the potentiometerdecreases. Voltage is coupled to the potentiometer through a pair ofconductors 90 and 92 and the control arm 88 of the potentiometer isconnected to the indicating system of FIG. 4 through a conductor 94, thepotentiometer 86 being shown schematically in that illustration.

The permissible ascent indicator 9 of FIGURE 1, as diagrammaticallyshown in FIGURE 4, is responsive to the signals generated by thepressure detectors 8, 8', 8" and provides an indication of the maximumascent or minimum depth permissible or the further descent anddecompression required to compensate for too rapid ascent. For thispurpose, the system comprises a voltmeter 100 having a scale withappropriate markings from 4 to +4, a power supply 102 connected throughan on-off switch 103 across a potentiometer 104, a power supply checkingnetwork 106 and a plurality of balancing impedance networks 108, 110 and112. The potentiometer 1041s adjusted to apply a voltage to an inputterminal 114 of the voltmeter 100 which corresponds to the voltageapplied to a second input terminal 116 of the meter when a balancedcondition exists in the balancing impedances 108, 110 and 112.

The power supply checking network 106 is provided for checking thecapacity of the power supply 102 and includes a pair of resistors 118and 120, a rheostat 122 and three switch contacts 124, 126 and 128,linked to operate in unison. When testing the supply 102, the contactsare moved to the positions opposite to those shown in the drawing,thereby placing the meter 100 across the power supply 102. Thereupon,the voltage of the supply 102 is indicated by the meter 100 and checkedfor its capability. This feature is provided in the decompressionmonitor because changes in the voltage of the supply 102 will produceproportional changes in the unbalanced condition indication.

The balancing impedance network 108 is electrically coupled to thebalance beam potentiometer 86 (shown diagrammatically in FIGURE 4), ofthe pressure balancing assembly of FIGURE 3 through the conductors 90and 92 and includes a pair of potentiometers 130 and 132 which regulatethe voltage developed across the potentiometer 86 for both the balancedcondition of operation and null or zero adjustment of the meter 100.Similarly, the networks 110 and 112 are associated with the pressuredetectors 8' and 8" of FIGURE 1, the balance beam potentiometers ofthese assemblies being diagrammatically shown as 86' and 86",respectively. These networks include two rheostats 134, 136 and 138,140, respectively, which regulate the voltage developed across thepotentiometers 86' and 86" for the balanced 6 condition of operation andthe null or zero adjustment of the meter 100.

The voltage developed across the potentiometer 86 is applied to arectifier 142 through its movable tap 8-8 and the conductor 94.Similarly, the voltages developed across the potentiometers 86 and 86are applied to a pair of diode rectifiers 144 and 146 through theirrespective movable taps 88 and 88". The cathodes of the diodes arecoupled together and to the input terminal 116 of the meter through thecontact 124.

In operation, when a diver wishes to ascend safely from a certain depth,the meter 100, which can be calibrated in feet of water, pounds persquare inch, fathoms, or the like, provides him with an accurateindication of how high he may ascend without exceeding the tissue ratioof the most susceptible body tissue or to what depth he must descendshould the maximum permissible tissue ratio be exceeded.

The on-otf switch 103 is thrown to the on position and the power supply102 is applied across each of the variable impedance networks 108, and112. As mentioned above, the effective resistance of any of the balancebeam potentiometers 86, 8-6 and 86" increases when the hydrostaticpressure of the liquid in its corresponding hydrostatic pressure chamber54 is greater than the pressure of the liquid in the pressure chamber 52divided by its specific tissue ratio, and decreases for the oppositecondition. It can be readily seen, therefore, that the balancingimpedance network corresponding to the pressure balance assembly havingthe greatest positive difference between the pressure of the liquid inthe accumulator pressure chamber 52 divided by its tissue ratio and thepressure of the liquid in the hydrostatic pressure chamber 54 willdevelop the largest voltage signal. The occurrence of such positivedifferences indicates an unsafe condition requiring the diver todescend. Similarly, the balancing impedance network corresponding to thepressure balance assembly having the least negative difference betweenthe pressure of the liquid in the tissue pressure chamber 52 divided byits respective tissue ratio and the pressure of the liquid in thehydrostatic pressure chamber 54 will develop the largest voltage signal,the negative pressure diflerence corresponding to a safe conditionpermitting ascent.

The voltage signals developed across the balancing potentiometers 86,86' and 86" are applied to the rectifiers 142, 144 and 146 through theirrespective movable taps 88, 88 and 88". Because the largest voltagesignal will back-bias the other two diodes and thereby preventconduction in them, only the largest of the three signals developedacross the impedances 86, 86' and 86" is detected at the input terminal116 of the voltmeter 100 through the normally closed contact 124 andcompared with the balanced condition voltage derived across thepotentiometer 104. When the arm of the meter 100 is deflected in thepositive direction, it indicates further descent to the indicated depthand decompression are required. When the arm of the meter 100 isdeflected in the negative direction, it indicates the minimal depth towhich the diver may ascend without exceeding the maximum permissibletissue ratio of the most susceptible body tissue.

Referring to FIGURE 5, there is shown one particular form of fluid flowbarrier which may be used as the barrier 22 in the accumulator assemblyof FIGURE 2. This barrier is designed to maintain constant for any givenpressure difference, the flow of a fluid between the diaphragm 16 andthe inner chamber 24 over a temperature range of 30 to 90 F. As notedabove, the time constant and rate of pressure buildup within the chamber24 are critical factors in effective simulation of a body tissue by theaccumulator assembly. Inasmuch as the time constant is partly dependentupon the porosity of the barrier 22 and the viscosity of the fluidemployed in the accumulator, the barrier of FIG. 5 serves to restrictfluid flow for increasing temperatures and liberate fluid flow fordecreasing temperatures.

The flow barrier includes a steel casing 150 having a hollow innerchamber 152, two cover plates 154 and 156 having internal recessescommunicating with the casing 150 through perforated corresponding steeldiaphragrns 158 and 160. The cover plates and diaphragms are mounted tothe casing 150 through corresponding gaskets 162 and 164, respectively,and the casing has two openings at opposite ends 166 and 168 which actas passageways between its inner chamber 152 and the diaphragm 16 andthe inner chamber 24, respectively, of the accumulator. Situated withinthe chamber 152 is an aluminum cylinder 170 which is centered within thechamber 152 and connected to the diaphragms 158 and 160 by a pair ofsupport screws 172 and 1'74, respectively.

In order to achieve the desired flow, the spacing between the cylinder170 and the inner surface of the inner chamber 152 must be very smalland the cylinder 170 must remain fairly well centered. This may beaccomplished by heating the aluminum cylinder 170 within the steelcasing 150 to a specified temperature, such as 660 R, which causes thealuminum cylinder 170 to yield such that, upon cooling to the operatingtemperature, the clearance between the cylinder 170 and the inner wallof the chamber 152 has the desired value. Thereafter, the casing 150 andthe cylinder 170 are reheated to a temperature at which the aluminumcylinder expands sufliciently to just engage the inner surface of thechamber 152. At this temperature, the diaphragms 158 and 160 and coverplates 154 and 156 are firmly attached. Upon cooling, the aluminumcylinder 170 will remain centered and have the proper clearance betweenthe top and bottom walls of the chamber 152.

The end thrust of the cylinder 170 due to a pressure differential acrossthe barrier is counteracted by maintaining the screws 172 and 174against the cover plates 154 and 156, respectively, when inserted intotheir respective receptacles. In this manner, the clearance between theheads of the screws 172 and 174 and the cover plates 154 and 156 is onthe order of the expected difierence in expansion in the axial directionbetween the drum 170 and the steel casing 150. Thus, any axial movementmay be minimized.

The rationale behind the design of the barrier of FIG- URE lies in thefact that liquids become more fluid with increasing temperatures.Normally, this would cause the flow between the diaphragm 16 and innerchamber 24 to increase, assuming a constant pressure differential acrossthe barrier 22. The aluminum cylinder 170, however, expands in size morethan the steel casing 150 for the same change in temperature causing theannular space existing between the cylinder and the top and bottom wallsof the inner chamber 152 to decrease in size with increasingtemperature. This tends to reduce the flow of the liquid. By properdesign, the decrease in viscosity is compensated by the decrease in theannular space between the cylinder 170 and the top and bottom walls ofthe inner chamber 152 such that the flow does not change with variationsin temperature.

Referring to FIGURE 6, there is shown an arrangement for employment asthe fluid flow barrier 22 in the accumulator assembly of the tissuesimulator 6 and also as the fluid flow barrier in the accumulatorassembly of either the tissue simulator 6 or the tissue simulator 6". Tothis end, there are provided a cylindrical steel housing 180 having aconstant diameter opening 182 formed therein and a pair of cover plates184 and 186, each cover plate similarly having a central recess formedtherein, bolted to opposite ends of the housing 180 by a plurality ofbolts 188. Interposed between the cover plates 184 and 186 and thehousing are a pair of gaskets 190 and 192, preferably formed of metal,which form a liquid tight seal between the cover plates 184 and 186 andthe housing 180. The housing 180 includes a passageway 194 which couplesthe opening 182 to conduit 196 leading to the diaphragm 16 of thepartial pressure chamber 14 and a pair of passageways 198 and 200 whichrespectively couple the opening 182 to a conduit 202 leading to theaccumulator chamber 24 of the tissue simulator 6 and to a conduit 204leading to the accumulator chamber 24' of the second tissue simulator6'. In the alternative, the conduit 204 may couple the passageway 200 tothe accumulator chamber 24 of the third tissue simulator 6".

Situated Within the opening 182 and extending slightly beyond the endsof the housing is a generally cylindrical member 206, preferably formedof metal. The member 206 is centered within the opening 182 by a pair ofspring clips 208 and 210 mounted in the recesses formed in the coverplates 184 and 186, respectively. The member 206 includes large diameterportions 212 and 214, of varying thickness, which separate a smallerdiameter portion 216 from a pair of smaller diameter portions 218 and220, respectively. The small diameter portions 218 and 220 may haveformed therein flattened grooves which receive spring clips (not shown)for forcing member 206 against the bottom wall of housing 180, providingpassageways of predetermined controlled size from portion 216 to thesmaller diameter portions 218 and 220, respectively. With the springclips 208 and 210 unbiased, the smaller diameter portion 216 issubstantially aligned with the passageway 194 and the smaller diameterportions 218 and 220 are substantially aligned with the passageways 198and 220, respectively.

When the pressure multiplier 4 (FIGURES l and 2) applies a pressureagainst the fluid in the partial pressure chamber 14, liquid flows fromthe diaphragm 16 through the conduit 196 and the passageway 194 and intoa central channel defined by the area between the wall of the opening182 and the thickness of the smaller diameter portion 216. From thecentral channel the fluid passes across the passageways formed by thelarge diameter portions 212 and 214 and wall 182 and into two sidechannels, the side channels being defined by the area between the wallof the opening 182 and the thickness of the smaller diameter portions218 and 220. The fluid then passes into the accumulator chambers 24 and24 through the passageways 198 and 200 and the conduits 202 and 204,respectively. It can be seen that the rate at which the fluid will passfrom the central channel to the two side channels is dependent upon thedifference in pressure between the fluid in the diaphragm 16 and thefluid which has already passed into the chambers 24 and 24'. Moreover,the rate is dependent upon the thickness of the large diameter portions212 and 214 which provide constricted passageways between the centralchannel and the two side channels. Accordingly, by varying the thicknessof the large diameter portions 212 and 214, the cylindrical member 206acts as two separate flow barriers, each barrier having a ditferent timeconstant, in the same manner as described in connection with FIGURE 5above.

From the above described illustrative embodiment, it can be seen thatthe decompression monitor contains several new features, among which arethe use of a balancing system to compare the partial pressure of thedissolved gas with the ambient pressure; the use of a pressure sensitivebellows to vary the tissue ratio as a function of the hydrostaticpressure; the simulation of a gaseous absorption by tissues of manydifferent time constants separately; the automatic selection of theparticular time constant most likely to produce decompression syndrome;the indication of the direction of magnitude of the hydrostatic pressuredeviation from the critical hydrostatic pressure; its adaptability toany type gas and percentage thereof in the breathing mixture; and theemployment of a temperature compensated flow barrier to maintain aconstant pressure buildup over a variable temperature range.

It is understood that the above-described invention is merelyillustrative and susceptible to considerable modification within theskill of the art. For example, Bourdon tubes or diaphragms may be usedinstead or with the bellows shown in the pressure balance assembly ofFIGURE 3 and electrical balancing may be used rather than mechanicalbalancing. Also, a mechanical system may be used to perform the sensingaction of the diodes in the indicating system of FIGURE 4. Such a sensormight consist of a spring-loaded arm which is actuated by the motion ofthe balance beam having the greatest imbalance. Moreover, in the barrierof FIGURE 6, the end clips 208, 210 may be eliminated, leaving only theclips pressing member 206 against the bottom wall. Accordingly, all suchvariations and modifications are included within the spirit and scope ofthe invention.

We claim:

1. A decompression monitor for automatically computing a decompressionschedule, comprising pressure accumulator means arranged to representthe gas absorption characteristics of a body tissue in response toapplied pressures and exposure duration thereto, including a fluid flowbarrier through which fluid flows at a rate proportional to thediflerence between the pressures on one side of said barrier and on theother side of said barrier, the one side being exposed to fluid at thepartial pressure of an inert gas to which the body is exposed and theother side being exposed to fluid at a pressure proportional to thepartial pressure of the inert gas to which the body is exposed and theexposure duration thereto, further comprising sensing means responsiveto the pressure accumulator means and to the ambient pressure forproviding a signal proportional to the difference between the ambientpres sure and representations of a critical pressure value as determinedby the pressure accumulator means, indicating means responsive to saidsignal for providing an indication of the difference, and pressuremultiplying means responsive to the ambient pressure and to a settingrepresenting the percentage of the inert gas in the breathing mixturefor applying a force to the fluid exposed to said one side of thebarrier which is proportional to the product of the ambient pressure andthe percentage of inert gas.

2. A decompression monitor for automatically computing a decompressionschedule, comprising pressure accumulator means arranged to representthe gas absorption characteristics of a body tissue in response toapplied pressures and exposure duration thereto, including a fluid flowbarrier through which fluid flows at a rate proportional to thedifference between the pressures on one side of said barrier and on theother side of said barrier, the one side being exposed to fluid at thepartial pressure of an inert gas to which the body is exposed and theother side being exposed to fluid at a pressure proportional to thepartial pressure of the inert gas to which the body is exposed and theexposure duration thereto, further comprising sensing means responsiveto the pressure accumulator means and to the ambient pressure forproviding a signal proportional to the difference between the ambientpressure and representations of a critical pressure value as determinedby the pressure accumulator means, and indicating means responsive tosaid signal for providing an indication of the difference, wherein thefluid flow barrier is responsive to temperature changes in inverserelation to the change of viscosity of the fluid with temperature toprovide a uniform flow rate with temperature for a given pressuredifference between the one side of said barrier and the other side ofsaid barrier.

3. A decompression monitor for automatically computing the decompressionschedule of an underwater diver comprising a plurality of pressureaccumulators for representing characteristics of different body tissues,each tissue being characterized by a maximum permissible ratio betweenthe partial pressure of dissolved gas to hydrostatic pressure, eachaccumulator being adapted to exponentially develop a pressureproportional to the percentage inert gas in the mixture breathed by saiddiver, the hydrostatic pressure and exposure duration thereto, aplurality of sensors, each sensor selectively coupled to a pressureaccumulator and responsive to the developed pressure therein and to thehydrostatic pressure for providing a force proportional to thedifference between the hydrostatic pressure and a critical hydrostaticpressure value as determined by the ratio between the developed pressureand the tissue ratio of the represented body tissue, and indicatingmeans responsive to the forces developed by the plurality of sensors forproviding an indication of the difference between said hydrostaticpressure and the critical hydrostatic pressure for the pressureaccumulator having the greatest positive difference between the criticalhydrostatic pressure and the hydrostatic pressure or the smallestnegative difference between said critical hydrostatic and hydrostaticpressures.

4. A decompression monitor as set forth in claim 3, wherein each of saidaccumulators includes a fluid flow barrier through which fluid flows ata rate proportional to the difference between the fluid pressure on oneside of said barrier and the fluid pressure on the other side of saidbarrier, the one side exposed to fluid at the partial pressure of theinert gas to which the diver is exposed and the other side being exposedto fluid at a pressure proportional to the partial pressure of the inertgas to which the diver is exposed and the exposure duration thereto.

5. A decompression monitor as set forth in claim 4, which furthercomprises a pressure multiplier responsive to the hydrostatic pressureand percentage inert gas in the breathing mixture for applying a forceto said liquid exposed to said one side of said barrier proportional tothe product of said hydrostatic pressure and percentage inert gas.

6. A decompression monitor as set forth in claim 4, wherein said fluidflow barriers are temperature responsive, expanding the flow area fordecreasing temperatures and constricting the flow area for increasingtemperatures.

7. A decompression monitor as set forth in claim 4, wherein each of saidplurality of sensors comprises a bellows arrangement responsive to thehydrostatic pressure and the pressure developed in its selectedaccumulator for converting said pressure into proportional staticforces, an adjustable balance for converting the static force exerted bysaid developed pressure into a force exerted by the critical hydrostaticpressure, and a bellows for varying the force exerted by the criticalhydrostatic pressure as a function of hydrostatic pressure.

8. A decompression monitor as set forth in claim 4, wherein each fluidflow barrier of said accumulators comprises an aluminum cylindersituated within the chamber of a hollow steel casing, said cylindersized so as to substantially decrease the annular spacing between saidcylinder and the top and bottom walls of said chamber with increasingtemperature and increase said spacing with decreasing temperature.

9. A decompression monitor as set forth in claim 3 wherein each of saidaccumulators includes a pair of fluid flow barriers through which fluidflows at a rate proportional to the difference between the fluidpressure on one side of the barriers and the fluid pressure on the otherside of the barriers, the one side of the barriers being exposed tofluid at the partial pressure of the inert gas to which the diver isexposed and the other side of the barriers being exposed to fluid at apressure proportional to the partial pressure of the inert gas to whichthe diver is exposed and the exposure duration thereto.

10. A decompression monitor as set forth in claim 9 which furthercomprises a pressure multiplier responsive to the hydrostatic pressureand percentage inert gas in the breating mixture for applying a force tothe liquid exposed to the one side of the barriers proportional to theproduct of the hydrostatic pressure and the percentage inert gas.

11. A decompression monitor as set forth in claim 10 wherein each of theaccumulators further include a partial pressure chamber responsive tothe force applied by the pressure multiplier for supplying liquid to oneside of the fluid flow barriers at a pressure proportional to theproduct of the hydrostatic pressure and the percentage inert gas andincludes a pair of accumulator chambers exposed to the other side of thefluid flow barriers for developing a pressure proportional to theproduct of the hydrostatic pressure and percentage inert gas and theexposure duration thereto.

12. A decompression monitor as set forth in claim 11 wherein the fluidflow barriers of each accumulator comprise a housing having an openingformed therein, a generally cylindrical member resiliently supportedwithin the opening, said cylindrical member having large diameterportions of varying thickness separating a smaller diameter portionleading to the partial pressure chamber from another smaller diameterportion leading to one of the accumulator chambers and still anothersmaller diameter portion leading to another one of the accumulatorchambers, each of the large diameter portions providing a constrictedpassageway between the smaller diameter portions for enabling fluid toflow at a controlled rate from the partial pressure chamber to bothaccumulator chambers.

References Cited FOREIGN PATENTS 735,170 5/1966 Canada.

DONALD O. WOODIEL, Primary Examiner US. Cl. X.R.

